C3 – Unit 7

Batteries

1. Batteries

Hey S3 click on ‘formal note’ below to get a copy of the unit notes. I’ve now coloured it and made it bold. I’ve put it between the dashed lines. What more can I do. Surely you can get! Doc F

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formal note

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The word battery is really an old military term for a group of guns or cannons that were fired in order. This provided continuous fire.

In electrical terms a battery is really a battery of cells. Several electrochemical cells put together to provide a continuous greater voltage.

Many of what we call “batteries” are just a single cell.

The battery contains chemicals which react to produce a voltage or current.

Research was carried out using datasheets on the Duracell website to look at a range of batteries and complete the table.

 

Name

Technology

Voltage
(V)

Dimensions
l x b x h (mm)

Use

MN908        
D357        
CR2450        
DC1500        
DL1/3N        
MN21        
NX2400        
625A        

 

There are a huge range of batteries for different uses.

They tend to be small, light and portable producing lower, safer voltages.

The chemicals inside the batteries are used up and they have to be replaced. The cost of the energy is high.

 

2. Rechargeable batteries

Two lead plates were placed in 2M sulphuric acid. A current was passed into this for two minutes.

Orsay

It was then connected to a light bulb which kept it lit for a minute or so.

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The “battery” was then charged again and kept the bulb lit again. This is a rechargeable battery.

This lead/acid technology is a very old technology and  is still used in car batteries, motorcycle batteries, etc.

 

3. Chemical reactions producing a voltage

A piece of magnesium and a piece of copper were connected to a meter an placed in a sodium chloride solution.

Orsay

This produced about 1.5 volts.

The sodium chloride is an electrolyte. It completes the circuit and allows ions to move. Electrons flow along the wires.

 

4. The Electrochemical Series

Different pairs of metals were connected to a voltmeter. Each metal was placed on either side of a piece of filter paper soaked in an electrolyte. Here the electrolyte was sodium chloride solution.

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Orsay

A table was drawn up to show the different voltages produced.

metal A

metal B

voltage (V)

copper

magnesium

1.41

iron

magnesium

1.24

zinc

magnesium

0.57

copper

zinc

0.78

iron

zinc

0.66

copper

iron

0.17

This was compared with page 7 of the Data Booklet. The further apart the metals are the greater the voltage. The current, or electrons, from from the metal higher up to the one lower down.

Half cells

The Chemistry is easier to control and explain if experiments are done in half cells.

Here one half cell is zinc in a zinc sulphate solution.

The other is copper in a copper sulphate solution.

The two half cells are connected to a voltmeter. There is no voltage.

When the half cells are linked with a ‘bridge’ soaked in ammonium sulphate solution a voltage is produced. The circuit is completed with this electrolyte and ions can move. It is known as an “ion bridge” or a “salt bridge”. A suitable electrolyte is chosen for the “ion bridge” that does not react with the solutions in either of the two half cells.

Orsay

Three different “ion bridges” were used –

1. paper towel soaked in ammonium sulphate solution

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2. cotton wool soaked in ammonium sulphate solution

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3. filter paper soaked in ammonium sulphate solution

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As in the previous experiments the electrons flow in the external circuit from the metal higher up the electrochemical series to the one lower down – ie. zinc to copper.

The experiment was repeated with the same solutions but allowing them to come in contact in a U-tube. A cotton wool plug kept the solutions from mixing.

Orsay

Again a voltage was produced.

The chemistry going on in the half cells is explained by ion-electron half equations. Again they are found on page 7 of the Data Booklet.

Zn2+ + 2e → Zn

and

Cu2+ + 2e → Cu

The zinc’s half equation has to be reversed as it has to produce the electrons. It is called an oxidation.

Zn → Zn2+ + 2e

The copper receives the electrons so it is not reversed. It is called a reduction.

 

CFE Outcomes

Level 4

Using experimental evidence, I can place metals in an electrochemical series and can use this information to make predictions about their use in chemical cells.

SCN 4-10a

Using a variety of sources, I have explored the latest developments in chemical cells technology and can evaluate their impact on society.

SCN 4-10b

 

National 4

Chemical cells and the electrochemical series

When different metals are connected by an electrolyte, an electric current flows from one metal to the other through connecting wires. By comparing pairs of metals the electrochemical series can be constructed. The electrochemical series is used to predict the size of voltage and direction of current in chemical cells. This forms the basis for batteries.